1. Field of the Invention
The present invention relates to multi-quantum well infrared photo-detectors, photo-sensors and image sensors which acquire various information by receiving infrared.
2. Description of the Related Art
Systems for acquiring various information on a target object by receiving infrared irradiated from the target object and generating infrared images are widely used. In the search for underground resources of the earth, for example, oil and mineral veins are searched by observing the distribution of infrared irradiated from the earth by an infrared sensor mounted on an artificial satellite.
Among infrared sensors, quantum well infrared photo-detectors (QWIP), which can change the excitation level of electrons by changing the width of a quantum well, can control detecting wavelength ranges. Therefore if information on a plurality of wavelength ranges is acquired by quantum well infrared photo-detectors having a plurality of detecting wavelength ranges, a higher order of information can be acquired in applications to fields which require more detailed information, such as infrared sensors for weapons guidance and defense systems.
FIG. 1 is a rough configuration of a conventional multi-quantum well infrared photo-detector having two detecting wavelength ranges. In this multi-quantum well infrared photo-detector, a common electrode 114 is disposed on a substrate 111, such as GaAs, and a first multi-quantum well layer MQW1 for detecting long wavelength infrared (LWIR) having a wavelength of 8-10 xcexcm is disposed thereon. Then on the first multi-quantum well layer MQW1, a second multi-quantum well layer MQW2 for detecting mid-wavelength infrared (MWIR) having a wavelength of 3-5 xcexcm is disposed via a contact layer 115. And a transistor T1 for reading signals is connected to the contact layer 115, and a transistor T2 for reading signals is connected to the electrode 116 on the second multi-quantum well layer MQW2.
When the quantum well infrared photo-detector receives infrared in a detecting wavelength range, electrons at the ground state level are excited to the excited state level, and electrons which transit to the conduction band from the excited state level increase, and if bias voltage is applied at this time, current flows.
In other words, if bias voltage is applied, current increases or decreases depending on the quantity of infrared to be received, therefore the multi-quantum well infrared photo-detector can measure the quantity of infrared to be received by measuring the value of the current. In this case, the multi-quantum well infrared photo-detector can be regarded as an impedance element which value changes depending on the quantity of infrared to be received.
The impedance value of the multi-quantum well layer corresponds to the energy difference between the ground state level and the excited state level, where impedance is small when the energy difference is small, and impedance is large when the energy difference is large. A long wavelength infrared has lower energy, therefore the energy difference between the ground state level and the excited state level of the multi-quantum well layer, which absorbs the long wavelength infrared, is smaller. A mid-wavelength infrared has higher energy, therefore the energy difference between the ground state level and the excited state level of the multi-quantum well layer, which absorbs the mid-wavelength infrared, is larger. As a consequence, the impedance R1 of the first multi-quantum well layer MQW1, which detects the long wavelength infrared (LWIR), is smaller, and the impedance R2 of the second multi-quantum well layer MQW2, which detects the mid-wavelength infrared (MWIR), is larger, and the difference between them is extremely large, where R1 less than  less than R2 establishes.
So, if a bias voltage V0 is applied from the common electrode 114, the transistor T1 is controlled to be conductive, and the transistor T2 is controlled to be non-conductive, then the bias voltage V0 is applied only to the first multi-quantum well layer MQW1, and the long wavelength infrared (LWIR) is detected (see FIG. 1B).
If the transistor T1 is controlled to be non-conductive and the transistor T2 is controlled to be conductive, on the other hand, the bias voltage is applied mostly to the second multi-quantum well layer MQW2, where the impedance R2 is large, and is barely applied to the first multi-quantum well layer MQW1, where the impedance R1 is small. The change of impedance caused by receiving infrared is primarily influenced by the second multi-quantum well layer MQW2, where the impedance R2 is large, therefore only the reception of mid-wavelength infrared (MWIR) can be detected.
In this way, a conventional multi-quantum well infrared photo-detector reads the series of signals in a time sequence corresponding to two detecting wavelengths, using the difference between impedances R1 and R2 of the two multi-quantum well layers MQW1 and MQW2.
The conventional multi-quantum well infrared photo-sensor reads signals of 8-10 xcexcm wavelengths of long wavelength infrared (LWIR) and 3-5 xcexcm wavelengths of mid-wavelength infrared (MWIR) due to the fact that the impedance of the respective multi-quantum well layers has about a two digit difference.
However, when a multi-quantum well layer which detecting wavelength is 8-9 xcexcm, and a multi-quantum well layer, which detecting wavelength is 9-10 xcexcm, are layered, the detecting wavelength regions are close to each other, and the difference of impedance between the multi-quantum well layers decreases. As a result, the signals of the two multi-quantum well layers are modulated by each other, and cross-talk between the two signals increases, making it difficult to acquire information for each different wavelength.
With the foregoing in view, it is an object of the present invention to provide a multi-quantum well infrared photo-detector which can acquire signals from respective detecting wavelength ranges independently without generating cross-talk, and which can also directly acquire sum signals and difference signals of each signal, even if a plurality of detecting wavelength ranges are close to each other.
To achieve this object, the present invention provides a multi-quantum well infrared photo-detector, in which a plurality of multi-quantum well layers having respective sensitivities for different wavelength ranges of infrared are layered via a common contact layer, comprising a switch where one end is connected to the above common contact layer, and a current integration unit which is connected to the other end of the above switch; wherein first and second voltages are applied to first and second contact layers at the opposite side of first and second multi-quantum well layers which are formed on and under the above common contact layer respectively, the above switch is conducted for a predetermined time so that either voltage between the above common contact layer and the first contact layer or voltage between the above common contact layer and the second contact layer becomes higher than the other, and the above current integration unit is charged or discharged by the current which flows in the above multi-quantum well layers.
According to the present invention, bias voltage can be applied individually to multi-quantum well layers having different detecting wavelength ranges by the first and second voltages applied to the electrodes and switch, therefore only signals of the multi-quantum well layer where the bias voltage is applied can be detected without generating cross-talk.